Check if column already has same cell value as in textbox

I'm making startlist so obviously I want to prevent that there would be competitors with same start number. I have TextBoxes for input values with Save button convert to DataGridView table. Like so:

bool CheckFiledIsEmpty(){if(textStN.Text==string.Empty||
        textN.Text==string.Empty||
        textSN.Text==string.Empty||
        textC.Text==string.Empty||
        textYB.Text==string.Empty)returnfalse;elsereturntrue;}privatevoid buttonSave_Click(object sender,EventArgs e){if(CheckFiledIsEmpty()){stringColumn1= textStN.Text;stringColumn2= textN.Text;stringColumn3= textSN.Text;stringColumn4= textC.Text;stringColumn5= textYB.Text;string[] row ={Column1,Column2,Column3,Column4,Column5};
        dataGridView1.Rows.Add(row);
        textStN.Text="";
        textN.Text="";
        textSN.Text="";
        textC.Text="";
        textYB.Text="";}else{MessageBox.Show("Input all information about competitor!");}}

I want that when the Save button is hit, the former values in Column1 (start number textbox textStN) are checked and if there is same value the error box like: "The competitor with that start number already exists!"

Thanks for help and sorry for my English.